State-of-the-Art Diesel Technology

Mercedes Benz has just released the OM651, a new four cylinder diesel engine with no less than 150kW and a massive 500Nm – from just 2.14 litres!

In the C-class 250 CDI BlueEFFICIENCY Prime Edition sedan, the NEDC fuel economy figure is just 5.2 litres/100km. The 0-100 km/h time for the 1645kg car is a claimed 7.0 seconds.

Gaining outputs of 70 kW/litre and 186 Nm/litre requires a range of sophisticated technologies. Let’s take a look.

Twin Turbochargers

The engine uses two series turbochargers integrated into a single compact module. These turbos comprise a small high pressure turbo and a larger low pressure turbo.

• Turbines

The high pressure turbine has a diameter of 38.5mm and is positioned directly in the exhaust manifold. The exhaust gases flow through this turbine first, causing it to rotate at speeds of up to 248,000 rpm. Integrated into this turbine housing is a bypass duct, which can be opened or closed by means of a charge-pressure control flap triggered by an actuator.

If the duct is closed, the whole exhaust stream flows through the high pressure turbine, allowing charge pressure to be built up at low engine revs.

As the engine speed increases, the charge-pressure control flap opens to prevent the high pressure turbocharger from becoming overloaded. A portion of the exhaust stream then flows through the bypass duct. Downstream from the high pressure turbine, the two exhaust gas streams join up again, and any remaining exhaust energy drives the 50mm low pressure turbine at a maximum speed of 185,000 revolutions per minute. The low pressure turbine uses a wastegate for exhaust gas bypassing.

Once the engine reaches medium revs, the high pressure turbine's charge pressure control flap is opened so wide that the high pressure turbine ceases to perform any appreciable work. This allows the full exhaust energy to be directed with low losses into the low pressure turbine, which then does all of the turbine work.

• Compressors

The two compressors are likewise connected in series and are in addition connected to a bypass duct.

The combustion air from the air cleaner first flows through the low pressure compressor (diameter 56.1 mm) where it is compressed as a function of the low pressure turbine's operating energy input. This pre-compressed air now passes into the high pressure compressor (diameter 41 mm) that is coupled to the high pressure turbine, where it undergoes further compression – the result is a genuine two-stage turbocharging process.

Once the engine reaches medium revs, the high pressure compressor can no longer handle the flow of air, meaning that the combustion air would heat up too much. To avoid this, the bypass duct opens to carry the combustion air past the high pressure compressor and directly to the intercooler for cooling. In this case, the charge pressure control flap is completely open too, meaning that the high pressure turbine is no longer performing any work. This is the equivalent of single-stage turbocharging.

Intercooler and EGR Cooler

A front-mount intercooler is fitted to reduce intake air temperatures from as high as 140 degrees C after the air leaves the compressors.

After the intercooler, an electrically controlled valve ensures precise regulation of the fresh air and recirculated exhaust gas. So as to optimise the quantity of exhaust gas recirculated and thereby achieve high recirculation rates, the exhaust gases are cooled down as required in a powerful heat exchanger with a large cross-sectional area.

Variable Intake Manifold

The combustion air subsequently flows into the intake manifold that supplies air to each cylinder in a uniform manner. Built into the manifold is an electrically controlled intake port shut-off which allows the cross-sectional area of each cylinder's intake port to be smoothly reduced in size. This alters the swirl of the combustion air in such a way as to guarantee that the charge movement in the cylinders is set for optimum combustion and exhaust emissions over the full range of engine loads and rev speeds.

Rear Cam Drive

So as to allow appropriate pedestrian impact requirements to be met when the engine is installed longitudinally, a rear-mounted camshaft drive is used.

The valve timing mechanism is another new development and reduces friction at the 16 intake and exhaust valves, which are controlled by one overhead intake shaft and one overhead exhaust shaft acting via cam followers featuring hydraulic valve clearance compensation. The camshaft, Lanchester balancer and the ancillary assemblies are driven by a combination of gearwheels and just a very short chain drive.

Fourth Generation Common Rail Injection

Fourth-generation common-rail technology is used. Rail pressure has been increased by 400 bar to 2000 bar, and new piezoelectric injectors featuring direct injector needle control allow more flexible injection timing. The maximum ignition pressure is 200 bar which also contributes to the high output. Twin airflow meters are used.

Production

The new four-cylinder diesel engine will replace four of the existing diesel engines at Mercedes-Benz. The engine can be installed either longitudinally or transversely. Mercedes-Benz is expected to produce up to 700,000 units per year.

Footnote: No CO2 emissions figures have been released by Mercedes Benz.

Turbos Revolutionising Diesels If you blinked when you saw that 233 Nm/litre figure, you won’t be alone in your surprise. This Mercedes Benz chart shows the evolution of specific power and torque outputs through their diesel engine progression – over the naturally aspirated OM601 series, the latest engine has nearly 3.8 times the specific torque and 2.6 times the specific power!